Acquisition Some slides from: Yung-Yu Chuang (DigiVfx) Jan Neumann, Pat Hanrahan, Alexei Efros Image Acquisition Digital Camera Film Outline • Pinhole camera • Lens • Lens aberrations • Exposure • Sensors • Noise • Sensing color Camera trial #1 scene film Put a piece of film in front of an object. source: Yung-Yu Chuang
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Pinhole camera - University of Haifacs.haifa.ac.il/hagit/courses/CP/Lectures/CP02... · 2010-04-02 · Depth of Field Small Aperture Large Aperture (Low speed) (High speed) Aperture
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Acquisition
Some slides from: Yung-Yu Chuang (DigiVfx) Jan Neumann, Pat Hanrahan, Alexei Efros
Add a barrier to block off most of the rays• It reduces blurring• The pinhole is known as the aperture• The image is inverted
barrier
pinhole camera
Modeling projection
• The coordinate system– Put the optical center (Center Of Projection) at the origin– Put the image plane (Projection Plane) in front of the COP– The camera looks down the negative z axis
• we need this if we want right-handed-coordinates
Modeling projection
• Projection equations– Compute intersection with PP of ray from (x,y,z) to COP– Derived using similar triangles (on board)
– We get the projection by throwing out the last coordinate:
In Homogenous Coordinates
• Projection is a matrix multiply using homogeneous coordinates:
Projection
Projection
http://users.skynet.be/J.Beever/pave.htm
Pinhole camera
scene film
Add a barrier to block off most of the rays• It reduces blurring• The pinhole is known as the aperture• The image is inverted
barrier
pinhole camera
Shrinking the Pinhole Aperture
Why make the aperture as small as possible?• Less light gets through• Diffraction effect
• Lens• Lens Aberrations• Exposure• Sensors• Noise• Sensing color
Adding a lens
scene film
Adding a lens
scene filmlens
“circle of confusion”
A lens focuses light onto the film• There is a specific distance at which objects are “in focus”• Other points project to a “circle of confusion” in the image
(Thin) Lens
• Any object point satisfying this equation is in focus
Thin lens equation:
Circle of Confusion
)'('
iiidb −=
• Blur Circle Diameter b : Derive using similar triangles
d
aperturediameter
io
'i'o
Blur Circle, baperture
Aperture controls Depth of Field
• Changing the aperture affects depth of field– Smaller aperture:
Near Lens Center Near Lens Outer Edge • Rays parallel to the axis do not converge
• Outer portions of the lens yield smaller focal lengths
Spherical aberration
Spherical aberration
Spherical lens are free of chromatic aberration but do not focus well.Parabolic lens does.
Spherical aberration
Astigmatism
Different focal length for inclined rays
Astigmatism
Change in size and shape of blur patches
Comapoint off the axis depicted as comet shaped blob
Reading: http://www.dpreview.com
Lens Glare
• Stray inter-reflections of light within the optical lens system• Happens when very bright sources are present in the scene
Outline
• Pinhole camera• Lens• Lens aberrations
• Exposure• Sensors• Noise• Sensing color
Exposure• Two main parameters:
– Aperture (in f stop)
– Shutter speed (in fraction of a second)
Shutter Leaf Shutter
• Advantages– Uniform illumination– Entire frame illuminated at once
• Disadvantages– Illumination not constant over time
– Limitations on shutter speed
Focal Plane Shutter
• Advantages– Cost effective (one shutter needed
for all lenses– Can achieve very fast shutter speeds
(~1/10000 sec)
• Disadvantages– May cause time distortion
Aperture• Aperture is the diameter of the lens opening,
usually specified by f-stop, f/D, a fraction of the focal length.– f/2.0 on a 50mm means that the aperture is 25mm– f/2.0 on a 100mm means that the aperture is 50mm
• When a change in f-stop occurs, the light is either doubled or cut in half.
• Lower f-stop, more light (larger lens opening)
• Higher f-stop, less light (smaller lens opening)
Aperture
Constant speed.
Shutter speed – Rule of Thumb
• 1 step in the shutter speed scale corresponds to 1 stop in the aperture scale.
• When a continuous scene is imaged on the sensor, the continuous image is divided into discrete elements - picture elements (pixels)
Spatial Sampling
0x
Sampling
• The density of the sampling denotes the separation capability of the resulting image
• Image resolution defines the finest details that are still visible by the image
• We use a cyclic pattern to test the separation capability of an image
Sampling Frequency
Sampling Frequency
1D Example:0
Nyquist Frequency
• Nyquist Rule: To observe details at frequency f (wavelength d) one must sample at frequency > 2f (sampling intervals < d/2)
•• The Frequency 2f is the NYQUIST frequency.
• Aliasing: If the pattern wavelength is less than 2d erroneous patterns may be produced.
Aliasing - Moiré Patterns Quantization
Digitizers (Quantization) Image Sensors• CCD
– Charge Coupled Device
• CMOS– Complementary Metal Oxide Semiconductor
MOS (Metal Oxide Semiconductor)
• Photosensitive element• Charge acquired depends on the number of
photons which reach the element• CCD devices are arrays of this basic element
Photoelectric Effect
photon phot
on
Hole Electron
Incr
easi
ng e
nerg
y
Valence Band
Conduction Band
1.26eV
• Thermally generated electrons are indistinguishable from photo-generated electrons ‘Dark Current’.
Quantum Efficiency• Not every photon hitting a pixel creates a free electron• Quantum Efficiency (QE) =
– electrons collected / photons hitting the pixel
• QE heavily depends on the wavelength
• QE < 100% degrades the SNR of a camera
• Typical max QE values : 25% (CMOS) … 60% (CCD)
pe SNRQESNR =
QE [%]
lambda [nm]
blue green red
CCD (Charge Coupled Device)
• Boyle and Smith, 1969, Bell Labs• Converts light into electrical signal (pixels)
CCD Readout
•Charge Amplifier
Bucket Brigade• Integration• Charge Shift and
Read-out
CMOS (Complementary Metal-Oxide Semiconductor)
• Each pixel owns its own charge-voltage conversion• No need for external shutter (electronic shutter)• The chip outputs digital bits• Much faster than CCD devices
CCD vs. CMOS• Mature technology• Specific technology• High production cost• High power consumption• Higher fill factor• Blooming• Sequential readout
• Recent technology• Standard IC technology• Cheap• Low power• Less sensitive• Per pixel amplification• Random pixel access• Smart pixels• On chip integration
with other components
Sensor Parameters
• Fill factor– The area in the sensor that is truly sensitive to light– Shift registers and others can reduce it up to a 30%
• Well capacity– The quantity of charge that can be stored in each pixel– Close relation with pixel dimensions
• Integration time:– Exposure time that is required to excite the CCD elements– Depends on the scene brightness
• Acquisition time:– Time needed to transfer the information gathered by the CCD– Depends on the number of pixels in the sensor
Fill Factor
• The ratio between the light sensitive pixel area and the total pixel area.
After color values are recorded, more color processing usually happens:
– White balance
– Non-linearity to approximate film response or match TV monitor gamma
White Balance
automatic white balancewarmer +3
Gamma Correction• Gamma correction applied by the converter
redistributes the pixel luminance values so that limited brightness range captured by the sensor is “mapped” to match our eye’s sensitivity. Gamma = 2.2 is a good match to distribute relative brightness in a print or in a video display.